This invention relates in general to mass spectrometers and in particular to time-of-flight mass spectrometers.
Time-of-flight (“TOF”) analysis has found widespread application because particle velocity, momentum, and mass can be determined from an experiment by constraining the appropriate parameters for the experiment. Time-of-flight mass spectrometers (“TOFMSs”) have the very desirable characteristic of high ion transmission, high repetition rate, good resolution and modest cost, which makes them very attractive as a mass sensitive detector in analytical instrumentation. Such applications were until recently somewhat hampered by the fact that most analytical ion sources produce continuous ion beams. The pulsed operation of a conventional TOFMS causes a rather low duty-cycle and TOFMS could not live up to its promises. For more detailed description of the state of the art of TOFMS, please see “The New Time-Of-Fight Mass Spectrometry,” by Robert J. Cotter, Analytical Chemistry News and Features, Jul. 1, 1999, pages 445A-451A.
It is desirable for an interface design between a continuous ion source and a TOFMS to overcome two problems. One is bringing the ions with as little spatial and kinetic energy spread as much as possible into the spectrometer for the purpose of achieving high mass resolution. The other is using as many of the ions supplied by the continuous source as possible without compromising on the first requirement so that a high duty-cycle can be achieved. Today, the preferred and highly refined solution to these problems is orthogonal acceleration (“OA). See “Time-of-Flight Mass Spectrometry,” R. J. Cotter, ACS Symposium Series 547. By OA, it is meant that the ion beam emanating from the ion source enters the TOF instrument at a right angle with respect to the flight axes of the ions in the spectrometer. This geometry allows a low spatial and kinetic energy spread to be achieved. The duty-cycle objective is met by expanding the width of the extraction region so that a larger fraction of the ion beam coming fro the source can be sampled. Active ion storage can be achieved by accumulation of ions in an ion guide connecting ion source and extraction region during the time an extracted ion packet disperses in the instrument.
In U.S. Pat. No. 5,396,064, Myerholt et al. describe a multiplexing procedure using a conventional TOF instrument in which an extraction region involving a pair of grids is pulsed and a cross-correlation is carried out numerically. This scheme, however, is still seriously impaired in practice by the difficulty of implementing a procedure using a pair of grids and parameters allowing for space focusing. A conventional space-focusing type of TOFMS is difficult to operate in a full multiplexing mode over an extended mass range. The pair of grids cannot be pulsed sufficiently rapidly to accomplish this objective because of the time it takes for ions to drift into the region between the grids, Moreover, this drift, of course, is mass dependent. For this reason, space focusing, which requires an extraction region defined by more than one grid, is undesirable.
None of the above-described TOFS schemes are entirely satisfactory for measuring ions. It is therefore, desirable to provide an improved TOFMS technique where the above-described difficulties are avoided.